This invention relates to a device for reducing noise in a composite video signal of a video reproducing device such as a video disc player and a video tape player, capable of improving video S/N (luminance signal S/N) and color S/N at the stage of a composite video signal.
In case of improving the video S/N and the color S/N of video disc players and video tape players, conventional devices as composite video signal processing use a noise reduction circuit utilizing a several sample delay (i.e., correlation in the horizontal direction) such as one shown in FIG. 5. In this circuit, input composite video signals of the NTSC system, for example, are digitalized in an A/D converter 1 by sampling at a frequency 4 fsc, which is four times as high as a color subcarrier frequency fsc. This digital signal (current signal) and a delay signal, which was delayed at a period of frequency 4 D fsc which is 4 D times as high as a color subcarrier frequency fsc, where D is an integer, in a delay circuit 2 in consideration of phase relation of the color subcarrier frequency, are added in an adder 3, and then the result of addition is divided into 1/2 in a divider 4 to average them.
This noise reduction circuit aims at reducing noise having no horizontal correlation by the averaging processing in consideration of horizontal correlation of video signals. In the circuits of this type, an edge having no horizontal correlation such as a vertical edge becomes inaccurate, so that improvements are made such as detecting vertical edges and avoiding the averaging processing in the detected vertical edges.
In the above mentioned noise reduction circuit, the noise reduction effect is high in a high frequency region but it is low in a low frequency region. This is obvious from the phenomenon that a reproduced outputs of such equipments as a laser disc player and a video tape recorder actually exhibits horizontal stripes made from red, orange, red . . . as shown in FIG. 6, not withstanding that the original signal exhibits a plain red color. The phenomenon that the irregular shading is more prominent in the vertical direction than in the horizontal direction indicates that a PM (phase modulation) noise of a chrominance signal is generated comparatively more frequently in the low frequency region.
In the circuit in FIG. 5, however, noise in the low frequency region can not be improved sufficiently and, therefore, the S/N of a chrominance signal cannot be improved effectively.
It is, therefore, an object of the invention to eliminate the above mentioned disadvantage and improve effectively video S/N (luminanee signal S/N) as well as color S/N of video disc players and video tape playback devices at the stage of a composite video signals.